Reflecting the ever increasing quantity of information, various high-level performance such as higher copying speed has been being required for the electrophotographic technology. Extremely high performance is also required for toners which are used in electrophotography. As particularly important properties among such performance, may be mentioned fixing property, offsetting resistance, blocking resistance, grindability and smoothening of marks.
Owing to the adoption of high-speed copying, the quantity of heat which is received from a fixing hot roll to fix a toner on a paper surface has been reduced compared with the heat quantity employed at the time of low-speed copying. A demand has hence arisen for a toner having good fixing property even at low temperatures. Conventional toners are however not fully satisfactory, because those having good low-temperature fixing property have insufficient offsetting resistance or develop the so-called blocking phenomenon, namely, agglomeration of toner particles during their storage and application.
On the other hand, toners having good offsetting resistance contain a resin having a high glass transition temperature and a large molecular weight. Upon production of a toner, grinding is performed after a resin, coloring agent and other additives have been mixed and then melted and kneaded in a kneader. Such a resin is known to reduce the grindability of the resulting toner, thereby adversely affecting the productivity of the toner.
It has been required to deposit a toner in a large amount on a paper surface in order to form marks of a satisfactory density, since the proportion of a resin contained in the toner is large with that of carbon black also contained in the toner. Deposition of the toner in such a large amount however results in rugged paper surfaces, whereby smooth feeding of paper sheets is prevented and paper jamming hence takes place upon copying. The smoothening of marks may be achieved by reducing the amount of a toner on a paper surface. This reduction to the amount of the toner however caused another problem that the density of marks is lowered and the marks become less legible. With a view toward improving this problem, it may be contemplated of increasing the proportion of carbon black in the toner so that the desired mark density may be achieved by using the toner in a smaller amount. Such a reduced proportion of the resin in the toner however leads to reduced fixing property, storability and offsetting resistance, no matter which one of conventional resins is used as the resin. This smoothening of marks is particularly important for double-sided copies which have recently found increasing utility. There is accordingly an outstanding need for the solution of the above problem.
Toners obtained in accordance with conventional techniques are each consumed in a large amount upon formation of marks on a paper surface. They are therefore accompanied, for example, by the following problems:
(a) The paper surface becomes rough and paper jamming occurs upon copying, especially, upon making double-sided copies.
(b) Although more copies can be made per unit time by increasing the copying speed, the amperage is small because of the use of the domestic power source and the available heat quantity is hence limited. Accordingly, the fixing is troubled at such a high copying speed. Any attempt of improvements to this trouble however results in reduced offsetting and blocking resistance, whereby high-speed copying becomes no longer feasible.
With a view toward providing solutions for these problems, various investigations have been made in order to develop a binder resin suitable for use in toners. Fully satisfactory binder resins have however been unknown to date.